The present invention relates to a tilt projection optical system, and more particularly to a tilt projection optical system suitable for an image projector that, for example, performs enlarged projection from a primary image plane to a second image plane while being located at an angled position.
Description of the Prior Art
Various image projectors have been proposed that enlarge and project the image displayed on a liquid crystal display or similar apparatus and that perform the enlarged projection from an angled direction so that the screen may be increased in size while the projector itself may be made compact. Specific examples of such devices include a device in which all of the optical elements of the projection optical system comprise reflective mirrors (Japanese Laid-Open Patent Application 10-111474), a device in which all of the optical elements of the projection optical system comprise refracting lenses (Japanese Laid-Open Patent Application 10-282451), and a device that has a projection optical system comprising a combination of reflective mirrors and refracting lenses (Japanese Laid-Open Patent Application 9-179064).
As proposed in Japanese Laid-Open Patent Application 10-111474, if all of the optical elements comprise reflective mirrors, the number of components may be reduced. However, because the reflective mirror does not have the freedom of color aberration correction, the arrangement of the color-synthesizing optical elements (such as three-faced color-synthesizing prisms) is restricted when the construction is such that colors are obtained via multiple liquid crystal display panels. In addition, while it is necessary to form the mirror using plastic in order to obtain a large-diameter curved mirror at a low cost, it is difficult to form a highly efficient reflective coating on the plastic surface. Consequently, if a plastic mirror is used in a projector capable of producing a high level of brightness, the temperature of the mirror increases and the reflective surface thereof deforms, resulting in deterioration in aberrations and durability. In particular, because mirrors close to the aperture are highly sensitive to errors, if a plastic mirror is used in a highly bright projector as any of the mirrors close to the aperture, the performance deterioration due to the deformation of the mirror caused by temperature change is a problem.
As proposed in Japanese Laid-Open Patent Application 10-282451, if all of the optical elements comprise refracting lenses, projection from an angled position may be achieved with optical elements having a relatively small area. However, because a large number of decentered lens groups is required, and some of the lenses thereof must be decentered to a large degree, it is difficult to hold the optical elements together. Where reflective mirrors and refracting lenses are combined, as proposed by Japanese Laid-Open Patent Application 9-179064, the number of decentered lens groups needed is smaller and the construction of the projection optical system is simpler. However, in order to perform projection onto a large screen, a mirror having not only a high power but also a very large area, which is difficult to manufacture, is needed.
An object of the present invention is to provide an improved tilt projection optical system.
In particular, an object of the present invention is to provide an easy to manufacture, high-performance tilt projection optical system that is sufficiently thin because projection is performed from an angle.
These and other objects are achieved by a tilt projection optical system having the following construction:
a tilt projection optical system that performs enlarged projection from a primary image plane on the reduction side to a second image plane on an enlargement side while being located at an angled position, and that includes, sequentially from the primary image plane, a refracting lens group, a bending mirror, and a group that includes at least one reflective surface having a negative power, wherein the construction is such that (i) the optical system after the bending mirror is rotated by approximately 90 degrees based on the bending of the light path by the bending mirror, (ii) no intermediate real image is formed between the primary image plane and the second image plane, and (iii) the radius of the circle that encompasses all the light rays involved in the image formation on the screen and that is parallel to the surfaces of each lens of the refracting lens group enlarges once and then converges in terms of its radius on the enlargement side from the aperture of the refracting lens group, and wherein the following condition (1) is met:
0.35 less than Rmin/Rmax less than 0.85xe2x80x83xe2x80x83(1) 
where,
Rmax: the maximum value of the radius of the circle that encompasses all the light rays involved in the image formation on the screen and that is parallel to the surfaces of each lens of the refracting lens group, the maximum value being obtained when such circle enlarges in terms of its radius once on the enlargement side from the aperture; and
Rmin: the minimum value of the radius of the circle that encompasses all the light rays involved in the image formation on the screen and that is parallel to the surfaces of each lens of the refracting lens group, the minimum value being obtained on the enlargement side from the surface at which the maximum value Rmax is obtained.